Breast cancer is a frequently diagnosed malignancy among women in the United States. Early detection of breast cancer can improve survival rates. Studies show that use of screening mammography can reduce lesion size and stage at detection, improving the prognosis for survival. Currently, mammography is a well-established imaging technique for early detection of breast cancer. Annual screening mammography is recommended by the American Cancer Society for all women over the age of 40.
Further improvement in screening mammography technology is needed. Studies showed that some breast cancer is missed. Further, despite improved radiographic criteria for differentiating malignant from benign lesions of the breast, misclassification of lesions can occur in everyday clinical practice. An objective for new technology development in detection and diagnosis of breast cancer is to help radiologists to improve both the sensitivity and specificity (i.e., improve breast cancer detection rate) while minimizing the number of benign cases sent for biopsy.
Mammography has been advanced dramatically with improvements in mammographic screen-film image quality, image acquisition and computer vision techniques including computer-aided diagnosis (CAD). At least one study showed that use of CAD improved the breast cancer detection rate. Further, radiology gradually started its transition from analog mammography to digital mammography. Some advantages of digital over analog are believed to be the improved overall contrast; and the ability to manually manipulate the contrast of images on a softcopy display.
The contrast in mammography has been viewed as an important criteria. Clinical acquisition of x-ray mammograms requires specific techniques in order to obtain high quality images. Attenuation differences between various structures within the breast contribute to image contrast. Due to the similar composition of breast structures and the physical manifestations of breast carcinoma, mammographic imaging must be substantially different from general radiographic imaging. Low-energy x-rays are required to enhance the contrast to differentiate between normal tissues and carcinoma. One way to increase image contrast is to increase radiation dose. Because of the absorption of low-energy x-ray is higher than high-energy x-ray, radiation dose becomes a concern for mammography. This requires that traditional screen/film for mammography has much higher contrast than general radiography for imaging head, chest, abdomen, and extremities.
Digital mammography in part addresses the limitation of screen/film system on contrast i.e., the limited dynamic range. However, because of the similar density of breast tissues and the concern of the radiation dose, the breast is imaged within a narrow range of the available dynamic range. There are other factors affecting the image quality including proper x-ray exposure, quality of soft copy display, the dynamic range of the display and proper image processing to optimally display the images.
Because of the role of contrast in the diagnostics accuracy in mammography, various techniques and methods have been developed to enhance the low-contrast image to help radiologists to better visualize the abnormalities. Methods have been developed for improving disease diagnosis using contrast enhancement weighted for different frequency contents in mammographic and digital portal images. For example, see commonly assigned U.S. Patent Published Application No. 2002/0181797 (Young) and U.S. Patent Published Application No. 2003/0091222 (Young).
The present invention is directed to processing mammographic images acquired digitally. The method is directed to enhancing the overall contrast of the images while preserving the detail contrast.